The present invention relates to a liquid crystal display device incorporating Thin-Film Transistors and other switching elements, for use in display devices such as computer and television devices, and to a method of manufacturing said liquid crystal display device.
In the past, liquid crystal display devices of the active matrix type (using Thin-Film Transistors (TFTs), etc.) or simple matrix type (Super-Twisted Nematic (STN), etc.) have been in use. It is well known that in all of these liquid crystal display devices, the light transmittance was dependent on the viewing angle, because the light passed through liquid crystals of different relative alignment states according to the angle from which the screen was viewed. In particular, such screens were difficult to see when viewed from the side. Accordingly, there has been much research seeking to improve these viewing angle characteristics.
FIG. 13 is a plan view showing the structure of one pixel in the active matrix substrate of the liquid crystal display element of Japanese Unexamined Patent Publication No. 5-273569/1993. As shown in FIG. 13, on the active matrix substrate, a Thin-Film Transistor 55 is provided adjacent to the intersection of a gate line 52 with a source line 53.
As shown in FIG. 14, the TFT 55 is made up of the following layered on a transparent insulating substrate 51a of glass or similar material: a gate electrode 56 which is connected to the gate line 52, a gate insulating film 57, and a semiconductor layer 58, which is made of amorphous silicon and which is layered on the gate electrode 56. On the semiconductor layer 58, in sections in order to partially cover the semiconductor layer 58, are n+ xe2x80x94Si layers 59, which serve as ohmic contact layers. On one n+ xe2x80x94Si layer 59 is a source electrode 60, which is connected to the source line 53, and on the other n+ xe2x80x94Si layer 59 is a drain electrode 61, which is connected to a pixel electrode 62.
The pixel electrode 62 is provided in the rectangular area bordered by the gate line 52 and the source line 53, and distributed on the pixel electrode 62 in islet form are transparent insulating films 63 made of SiNX, SiO2, or similar material. Each transparent insulating film 63 also serves as a protective film. The active matrix substrate is completed with the covering of TFT 55, pixel electrode 62, and transparent insulating films 63 with an alignment film (not shown).
On a transparent insulating substrate 51b, provided opposite the active matrix substrate, are layered a counter electrode 65 and an alignment film (not shown), in that order. The liquid crystal display element is completed by filling the space between the two substrates with a liquid crystal 66.
In a liquid crystal display device of this structure, at axe2x80x2, where the transparent insulating films 63 do not cover the pixel electrode 62, the image signal voltage is applied directly to the counter electrode 65, but at bxe2x80x2, where the transparent insulating films 63 are provided, a divided capacitance voltage is applied, because the image signal voltage is applied through the serial capacitance of the capacitance of the liquid crystal 66 and the capacitance of the transparent insulating film 63.
In this way, two domains are created within one pixel, each of which applies a different voltage to the liquid crystal 66, resulting in a different light transmittance of the liquid crystal 66 within each domain. Accordingly, viewing angle characteristics when viewing the screen from the side can be improved. Further, by forming a tapered section cxe2x80x2 around the edges of the transparent insulating films 63, a clear image without rough edges can be obtained.
However, a disadvantage of the conventional structure outlined above is that the inclusion of the transparent insulating films 63 on the pixel electrode 62 creates a plurality of areas that have uneven surfaces, resulting in disturbance of alignment and deterioration of display characteristics. The thicker transparent the insulating films 63 are made (in order to increase the difference between the voltage applied at area axe2x80x2 and that applied at area bxe2x80x2), the more likely the disturbance of alignment. Further, another problem with increasing the thickness of the transparent insulating films 63 is that gap control becomes difficult, because the cell gap at area axe2x80x2 is different from that at area bxe2x80x2.
In order to solve these problems, Japanese Unexamined Patent Publication No. 7-175037/1995, shown in FIG. 15, disclosed a liquid crystal display device which would prevent the deterioration of alignment by providing a thicker alignment film 67 on the active matrix substrate side, and by making the interface between the alignment film 67 and the liquid crystal 66 a flat surface.
However, in making the surface of the alignment film 67 flat in order to prevent the disturbance of alignment, a minimum film thickness of approximately 0.5 xcexcm (=500 nm) is necessary, but the applied voltage must be increased, leading to the problem of increased power consumption. Further, the polyimide generally used for the alignment film 67 is not completely colorless, thus decreasing the display quality.
In general, the active-matrix-type liquid crystal display devices have had a comparatively wide viewing angle with good display quality, but the simple-matrix-type liquid crystal display devices have had a narrower viewing angle. For this reason, users selected the type of liquid crystal display device according to their needs, using the active-matrix-type devices having wider viewing angles, for example, for regular use in the office and elsewhere, or for presentations, while using the simple-matrix-type devices having narrower viewing angles in situations calling for privacy, for example, when preparing documents aboard aircraft or elsewhere in public. However, since there were cases when the same liquid crystal display device was used both in the office and aboard aircraft and elsewhere, purchasing a device with a viewing angle suited for one type of use created inconveniences when using the device in other ways.
Japanese Unexamined Patent Publication No. 6-59287/1994, discloses a liquid crystal display device which can meet both types of needs with one device. This liquid crystal display device, as shown in FIG. 21, controls the viewing angle using a TN-type liquid crystal display panel 151 and, for viewing angle control, a guest-host-type liquid crystal panel 152. In concrete terms, when using the device in the office or for a presentation, no voltage is applied to the guest-host-type liquid crystal panel 152, thus scattering the light and enabling a wide viewing angle (see FIG. 21(a)); when using the device aboard an aircraft or where viewing of screen images by others is unwanted, a voltage is applied to the guest-host-type liquid crystal panel 152, allowing the light to be transmitted in one direction only, increasing the parallelism of the backlight, and thereby enabling a narrow viewing angle (see FIG. 21(b)).
However, since this liquid crystal display device uses two liquid crystal panels, the thickness and weight of the device as a whole is increased, as are costs. Further, power to drive the two liquid crystal panels is necessary, as well as power to ensure that the backlight is not dimmed due to passing through the two liquid crystal panels, resulting in the problem of increased power consumption. For these reasons, the liquid crystal display device discussed above could not be used for laptop-type personal computers or other portable information terminals.
The primary object of this invention is to provide a liquid crystal display device which enables widening of the angle of visibility by effectively forming within a single pixel domains with different viewing angle characteristics, but without giving rise to disturbance of alignment.
Further, the secondary object of this invention is to provide a liquid crystal display device which enables change of the device""s angle of visibility in a low-power-consumption, thin, lightweight, low-cost structure.
In order to attain the primary object mentioned above, the first liquid crystal display device of the present invention includes a first substrate having switching elements, scanning lines which transmit driving signals to drive the switching elements, signal lines which transmit image signals to the switching elements, and pixel electrodes connected to the switching elements; a second substrate, having counter electrodes, provided opposite the first substrate; and liquid crystal filling the space between the first substrate and the second substrate. The pixel electrodes are provided in two or more layers with insulating film between each pair of layers, and holes are provided in the pixel electrodes on the uppermost layer opposite the pixel electrodes on the lowest layer.
With the above structure, since the switching element is connected to each pixel electrode, the image signal voltage From the switching element is applied to each pixel electrode. Where there are no holes provided in the pixel electrode on the uppermost layer, the image signal voltage is applied directly to the liquid crystal between the pixel electrode on the uppermost layer and the counter electrode opposite it. On the other hand, where there is a hole provided in the pixel electrode on the uppermost layer, the inter-layer insulating film and the liquid crystal fall between the counter electrode and the pixel electrode on the lowest layer, and therefore the divided capacitance voltage is applied, where the serial capacitance of the capacitance of the liquid crystal and the capacitance of the inter-layer insulating film is applied.
Accordingly, two or more domains where the voltage applied to the liquid crystal differs are provided within a single pixel, and as a result two or more domains where the light transmittance of the liquid crystal differs are provided within a single pixel. This improves the viewing angle characteristics when the screen is viewed from the side.
Further, in the conventional structures, in order to form the domains where the voltage applied to the liquid crystal differs, in accordance with the thickness of transparent insulating films distributed on the pixel electrode in islet form, the voltage was adjusted and applied to the areas covered by the transparent insulating films and the areas not so covered, thus resulting in unevenness on the pixel electrode as thick as the transparent insulating films. However, in the present invention, if the thickness, the dielectric constant, and the area (equal to the area of the holes in the upper-layer pixel electrode) of the inter-layer insulating film are adjusted, the voltage applied to the liquid crystal can be regulated in accordance with the mere presence or absence of the pixel electrode on the uppermost layer, thus creating unevenness only as thick as the pixel electrode on the uppermost layer. By means of this structure, the pixel electrode on the uppermost layer becomes flatter, and the disturbance of alignment can be prevented.
The method of manufacturing the first liquid crystal display device includes the steps of: (a) connecting, on the first substrate, respective switching elements and multiple lowest-layer pixel electrodes so as to be provided in the form of a matrix, and providing scanning lines and signal lines so as to cross one another and be connected to the switching elements; (b) providing, on the lowest-layer pixel electrodes, at least one layer of upper-layer pixel electrodes, electrically connected to the switching elements, with inter-layer insulating film between each pair of layers; (c) providing holes in the pixel electrodes on the uppermost layer so as to be opposite the lowest-layer pixel electrodes; and (d) filling the space between the first substrate and the second substrate opposite it with liquid crystal.
According to this method, the first liquid crystal display device can be prepared easily and with a minimum increase in costs.
Next, in order to attain the secondary object, the second liquid crystal display device of the present invention includes a first substrate having switching elements, scanning lines which transmit driving signals to drive the switching elements, signal lines which transmit image signals to the switching elements, and pixel electrodes connected to switching elements; a second substrate, having a counter electrode, provided opposite the first substrate; and liquid crystal filling the space between the first substrate and the second substrate. The pixel electrodes are provided in two or more layers with insulating film between each pair of layers, holes are provided in the pixel electrodes on the uppermost layer so as to be opposite the pixel electrodes on the lowest layer, and the viewing angle is changed depending upon whether a common image signal is applied to the pixel electrodes on all the layers, or only to the pixel electrodes on the lowest layer.
With the above structure, when the switching element is driven by the driving signals from the scanning line, the image signal from the signal line is sent to the pixel electrodes through the switching element. Thus, the voltage is applied to the liquid crystal between the pixel electrode and the counter electrode.
If a common image signal is applied simultaneously to the pixel electrodes on all the layers within one pixel, in the places where there are no holes in the pixel electrode on the uppermost layer, the image signal is applied directly to the liquid crystal between the pixel electrode on the uppermost layer and its counter electrode. In the places where there is a hole provided in the pixel electrode on the uppermost layer, however, the inter-layer insulating film and the liquid crystal fall between the counter electrode and the pixel electrode on the lowest layer, and therefore the divided capacitance voltage is applied, where the serial capacitance of the capacitance of the liquid crystal and the capacitance of the inter-layer insulating film is applied. Accordingly, two or more domains where the voltage applied to the liquid crystal differs are provided within a single pixel, and as a result two or more regions where the light transmittance of the liquid crystal differs are provided within a single pixel. This improves the viewing angle characteristics when the screen of the liquid crystal display device is viewed from the side, thus enabling a wide viewing angle.
If, on the other hand, the image signal is applied only to the pixel electrodes on the lowest layer, the voltage equal to the divided capacitance voltage is applied to the entirety of the liquid crystal within each pixel, and therefore the light transmittance of the liquid crystal is the same throughout the pixel, thus enabling a narrow viewing angle.
As a result, it becomes possible to change the viewing angle without providing two liquid crystal panels, as was necessary with the conventional structures, and thus provision of a low-cost, thin, lightweight, low-power-consumption liquid crystal display device becomes possible. Accordingly, it can be used in laptop-type computers and other portable information terminals.
Further, the secondary object may be attained by the third liquid crystal display device of the present invention. This device has a structure comprised of a first substrate having switching elements, scanning lines which transmit driving signals to drive the switching elements, signal lines which transmit image signals to the switching elements, and pixel electrodes connected to the switching elements; a second substrate, having counter electrodes, provided opposite the first substrate; and liquid crystal filling the space between the first substrate and the second substrate; where the pixel electrodes are provided in two or more layers with insulating film between each pair of layers, and holes are provided in the upper-layer pixel electrodes and in the inter-layer insulating film immediately beneath each upper-layer pixel electrode so as to be opposite the pixel electrode on the lowest layer, and the viewing angle is changed depending upon whether respective different image signals are simultaneously applied to the pixel electrodes on the different layers, or a common image signal is applied simultaneously to the pixel electrodes on all the layers.
With the above structure, if different image signals are applied simultaneously to the pixel electrodes on different layers within one pixel, in the places where there are no holes in the upper-layer pixel electrodes and the inter-layer insulating film immediately beneath them, the image signal is applied directly to the liquid crystal between the pixel electrode on the uppermost layer and its counter electrode. In the places where there is a hole provided in the upper-layer pixel electrodes and the inter-layer insulating film immediately beneath them, however, the image signal is applied directly to the liquid crystal between the pixel electrode on the lowest layer and its counter electrode. Since the image signals applied to the pixel electrodes on the upper and lowest layers differ, two or more domains where the voltage applied to the liquid crystal differs are provided within a single pixel, and as a result two or more domains where the light transmittance of the liquid crystal differs are provided within a single pixel. This improves the viewing angle characteristics when the screen of the liquid crystal display device is viewed from the side, thus enabling a wide viewing angle.
If, on the other hand, a common image signal is applied to the pixel electrodes on the different layers, the same voltage is applied to the entirety of the liquid crystal within the pixel, and therefore the light transmittance of the liquid crystal is the same throughout the pixel. This enables a narrow viewing angle.
Thus, it becomes possible to change the viewing angle without providing two liquid crystal panels, as was necessary with the conventional structures, and thus provision of a low-cost, thin, lightweight, low-power-consumption liquid crystal display device becomes possible.
The other objects, features, and superior points of this invention will be made clear by the description below. Further, the advantages of this invention will be evident from the following explanation which refers to the Figures.